[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

My ten-year-old take on Alaskan hadrosaurs



I wrote this in 1989 as lark, and possibly as an exploration of my own
future research (I was a grad student of Ostrom's).  By now, obviously,
much of it needs to be updated. Despite that, it might shed some light
(ha!) on Alaska's hadrosaurs.  Sorry it's so long.

Feel free to tear it apart.  It's really all just speculation, but I think
it's got a solid basis - of course, look at me now.  :)

If any reader wants the digestive and ecological formulas, ask Jim Farlow. :)


The Colville River Site

The Colville River fossil locality, discovered in 1961, has been
extensively studied during the last five years. Located on Alaska's North
Slope, it is of early Maastrichtian or possibly late Campanian age. The
fossils are found in five distinct bone beds in a matrix of loose brown
siltstone, rich in terrestrial carbon, derived from soft sandy sediments
which may be the actual paleosoil (Brouwers et al 1987).

Animal life is represented by several hadrosaurs, tyrannosaurid teeth, one
Troodon tooth, and several invertebrates. All are well-preserved and
undistorted. Plant remains are prevalent and include at least seven genera.

The fossils are sorted by size - the larger and denser objects lie at the
base of the beds. Two of the five beds have been determined to be fluvial
channel lag deposits (ibid.) but currents are not likely to be responsible
for the vertical stratification. This may be due to secondary sorting in
stagnant water or down a steep underwater slope. The relationship of the
teeth and the three non-current beds has not yet been published.

Not unlike other hadrosaur finds in Alberta, Montana, and Wyoming, the area
is thought to have been a coastal plain next to a shallow sea (Davies
1987). An extensive number of the invertebrates found at the site are known
to have occupied low-energy environments (Brouwers et al 1987). The region
was probably a swamp or river delta at the time of deposition.


Vertebrate Remains

At least seven individual hadrosaurs have been identified, the largest of
which was 9-10 metres long (ibid.). Most are only 3.5 to 4.5 metres long
with neural arches remaining unfused to the vertebrae (Davies 1987). These
are juveniles, still growing.

No cranial remains are present, but it is thought the hadrosaurs are
lambeosaurine. Teeth are the most durable portions of vertebrate skeletons
(Behrensmeyer and Boaz 1980); this is especially true for the massive
dental batteries of hadrosaurs. Were the heads present on burial, the teeth
would have been preserved at least as well as the rest of the remains.
These animals appear to have lost their heads before deposition.

Horner (1984) and Currie (1983) have hypothesized that sub-adult hadrosaurs
regularly congregated in herds apart from the sexually mature adults, but
that could be contested if the juveniles at Colville River are associated
with the lone adult.

The Troodon tooth may paint members of that genus as scavenging omnivores
(Brouwers et ail 1987), yet the Colville River Troodon tooth is a
taphonomic dilemma. It is not associated with the hadrosaur remains, and
may have washed down from the paleo-Alaskan highlands. If so, this is
important not because of what it implies for Troodon feeding strategies,
but because it tells us that dinosaurs lived in the Alaskan highlands,
where temperatures probably dropped below freezing in winter.

The presence of large theropod teeth may indicate that feeding took place
at the site of burial, but they also may have been carried to the site by a
current. Considering the excellent state of preservation, a SEM analysis of
the bone surfaces and the tooth wear might provide answers.


The Late K Alaskan Climate

Modern Alaska's North Slope challenges an organism to survive. The
challenge can be broken down into two elements: low temperature and an
exaggerated solar cycle. During the Maastrichtian, Alaska reached 18
degrees above the Arctic Circle (Brouwers et ail 1987), north enough to
allow considerable seasonal variation in hours of sunlight. All organisms
present throughout the year would have had to adapt to survive the dark
winters.

Maastrichtian Alaska was warmer than it is today. Barron (1981) thinks the
polar regions averaged five degrees C in winter and 19 degrees C in summer.
Colville River invertebrates indicate water temperatures between two and 12
degrees C and the plants are indicative of a mild to cold temperate forest
(Brouwers et ail 1987). Axelrod (1984) used O2 isotope ratios to derive a
late K polar sea temperature of about 17 degrees C. For Colville River, he
postulates an average January temperature of ten degrees C and an average
July temperature of 20 degrees C. He draws a parallel between the climate
of the Colville River site and the modern temperate rain forests of the
Sierra Madre Oriental, with its warm rainy summers and cool dry winters.
Spicer and Parrish (1986) use west-central Oregon as an example. Whatever
the exact numbers, there is no doubt it was warmer than today.

The combination of an Arctic light cycle and warm climate is foreign to
human experience. How did the combination affect plants? Greenhouse
experience suggests that plant productivity would skyrocket, at least
during the summer.


Flora of the Colville River Site

Plant remains, excellently preserved, consist of herbaceous debris and
roots. Four distinct environmental communities are found: coastal
low-ground; coastal high-ground; aquatic; and inland.

Ferns and small herbaceous angiosperms occupied low, wet micro-environments
on the coastal plain. In the drier high-ground grew conifers such as
Metasequoia. Roehler and Stricker (1984) describe another site from the
mid-K where conifer stumps and logs are interspersed with hadrosaur skin
impressions and footprints of smaller dinosaurs. They found no skeletal
remains.

The inland forest is the best-studied of the habitats. Seed ferns,
deciduous and evergreen conifers, deciduous and evergreen cycadophytes, and
deciduous gingkos all grew abundantly. These genera are found in mild to
cold temperate forests and the list of flora is almost identical to the
contemporaneous paleo-flora of Alberta, Montana, and Wyoming. A single
coastal forest may have spread from Alaska as far south as Montana (Wolfe
and Upchurch 1987, Brouwers et ail 1987).

Even were there a single forest stretching from Alaska to Montana, the
northernmost flora would have had distinct adaptations due to the light
stress incurred at such high altitudes and, in fact, the Alaskan flora were
probably separate species. The Alaskan plants exhibit a high degree of
deciduous behavior in taxa that are generally green throughout the year.
Deciduousness is a widespread adaptation to seasonal stress. Axelrod (1984)
estimates that sunlight would have been negligible for two months each year
at Colville River.

But if winter is dark, then summer is light. The productivity of plants
with 20 hours of usable light each day at 20 degree C would be tremendous
(Hotton 1980), possibly far beyond any present environment.


The Good Stuff: Alaskan Hadrosaur Ecology and Behavior

Hadrosaurs and at least two other types of dinosaur lived on Alaska's North
Slope. At least during the summer, enough vegetation grew to feed a viable
population of the large herbivores. The dominant plants were for the most
part deciduous, having adapted to two months of dark (but not necessarily
cold) each winter. A parallel degree of annual dormancy is likely in the
small angiosperms.

Individual hadrosaurs would have been hard-pressed to find high-quality
forage during those lean months, and herds would multiply the difficulty.
Three options are available to the animals: overwinter while remaining
active, overwinter while dormant or with curtailed activity; or migrate to
a more hospitable environment.

Were the Alaskan hadrosaurs active all winter, they risked destruction.
Animals would have had to survive those two months by eating the remaining
living matter, eating dead plant matter, or living off metabolic reserves.
The effect of a single poor decade or even a single crippling year for the
plants, combined with the undoubtedly huge appetites of the hadrosaurs
would have crippled the plant population, and thus the dinosaurs as well.

A more stable strategy for overwintering hadrosaurs to adopt would be
hibernation. Estivation is as widespread in the animal kingdom as
deciduousness is in plants. The physiological capability of hadrosaurs to
hibernate should not be in doubt. Extant alligators have been known to
hibernate for over a year and can withstand temperatures as low as four
degrees C (Axelrod 1984). Hibernating hadrosaurs would have had to possess
either some degree of thermoregulation or a natural antifreeze as
temperatures in a climate with a mean temperature of five degrees C would
occasionally have dropped below freezing.

Brouwers et ail (1987) cite the presence of juvenile hadrosaurs as evidence
they overwintered in Alaska as part of multi-age groups. Bakker (in a
Discovery magazine article, Morrell 1987) estimates that hadrosaur
hatchlings grow to three metres in one year. Assuming early hadrosaur
growth is roughly linear with respect to time and that Bakker is correct,
the smaller Colville River hadrosaurs died during their second year.

Migration is the most intriguing hypothesis of all. Axelrod (1984)
concludes that at an average speed of 15 km/day, an organism or herd could
cross ten degrees of latitude, equal to the north-south length of the
province of Alberta, in four months. Hotton (1980) thinks three times that
distance could be covered in four to six months. Assuming a mere 6 hours of
activity each day, Hotton's estimate is only as fast as the slowest
dinosaur speed yet inferred from preserved trackways (belonging to a
sauropod). There is no temporal barrier to migrating dinosaurs.

Researchers don't know how hadrosaurs walked. Footprint evidence shows they
walked on two legs and on four, but the preferred mode and the feeding
posture remain unknown. Some scientists postulate that hadrosaurs
habitually walked and fed on two legs, occasionally dropping down on all
fours (Maryanska and Osmalska 1983), yet the mani end in blunt hooves whose
impressions trackways often preserve (Currie 1983). Bakker (1986) advocates
a purely quadrupedal stance, but hadrosaurs were at least facultative
bipeds and probably used that ability when foraging. It would enable them
to exploit food sources out of reach of ceratopians.


Four Legs Good, Two Legs Better?

Taylor (1973) discovered that the energetic cost of running was the same in
two facultative bipeds (chimpanzee and capuchin monkey) whether they ran on
two legs or four. More recent examples have studied the costs of walking,
more appropriate since migrating terrestrial animals walk instead of run
(Pennycuick 1975).

In birds and humans, bipedalism provides one significant advantage over
quadrupeds of similar size (Cavagna et ail 1976, 1977). Bipeds can more
effectively recover and store the applied mechanical energy of each stride
as potential energy. Humans walking at four km/h recover 65-70 % of the
energy they exert. Walking birds post similar results, while quadrupeds can
manage no better than 50 % return. This translates into increased
endurance, and greater distances can be covered by a biped than by a
quadruped of equivalent mass for a given energy output (Carrier 1984).

Another approach can be used to test the validity of the migration
hypothesis. Hadrosaur communal nesting (Horner 1982, 1984) would lead to
heavy foraging around the nesting location, unless the adults ceased
feeding. If hadrosaurs did not migrate and such large-scale feeding took
place over a limited range throughout each year, the local flora would be
devastated. Modern African elephant herds eat so much, they must
continually move or they would starve (Engelmann 1966). Late K dinosaur
communities, especially nesting colonies, would likely have had the same
constraints. Seasonal migration would allow food sources to recover until
the next breeding season.


Hadrosaur Guts, Served with a Grain of Salt

Large terrestrial herbivores have less selective diets and consume greater
amounts of poor quality vegetation than do smaller animals (Mellett 1983,
Ostrom 1980, Farlow 1987). African elephants regularly eat bark, entire
shrubs, and even chunks of wood in addition to leaves and grass (Engelmann
1966). Hadrosaurs probably had a similar diet; they sported massive dental
batteries suited to the task (Ostrom 1964). The fossilized stomach contents
of one included pine needles and twigs (Beland and Russell 1978).

Troyer (1984) elegantly showed that Iguana iguana exhibits a digestive
efficiency equivalent to that of a herbivorous mammal of equal mass. If
herbivorous dinosaur-reptile-mammal digestive efficiencies do not change
across taxa for a given size, we can begin to speculate on the foraging
behavior and digestion of large herbivorous dinosaurs.

The assimilation efficiency (AE = percent of ingested plant material that
is digested) of large herbivorous mammals is well-studied. Estimates of
elephant AE range from 44% (Engelmann 1966) to 32% (Colinvaux 1986). Cattle
AE ranges from 40% (Petrides and Swank 1965) to 38% (Colinvaux 1986).
Smaller mammals, eating higher quality food, have higher efficiencies
(Colinvaux 1986). If large dinosaurs, mammals, and reptiles all have the
same AE, we can conservatively guess that hadrosaurs had an AE of about 40%.

Using experimental data, Farlow (1976,1980) and Coe (1976) derived formulae
which can be used to extrapolate the basal metabolic rate (BMR = calories
used by an animal at rest) of mammals, birds, and reptiles (at a body
temperature of 30 degrees C) from their mass. An 8,000 kg mammal has a
predicted BMR of 2.2 x 10^7 kcal/yr, a bird 2.0 x 10^7 kcal/yr and a
reptile 5.4 x 10^6 kcal/yr.

Basal ingestion (IB = calories an animal must eat to maintain body weight)
can also be derived from an animal's mass. Different formulae must be used
for ectothermic and endothermic organisms. An 8,000 kg hadrosaur needs to
eat 4.0 x 10^7 kcal/yr (= IB) if it is endothermic, and 75% less if it is
not. In order to best constrain this analysis, further calculations will
only consider hadrosaurs to be endothermic.

IB (the number of calories eaten) multiplied by AE (the % digestive
efficiency) gives gross production (GP = calories or carbon absorbed by the
gut of an animal). An AE of 40% gives a GP of 1.6 x 10^7 kcal/yr,
approximately equal to the predicted BMR of warm-blooded birds and mammals.
If we work the other way, towards the AE from the predicted BMR, we derive
an AE of 55%, high for large mammals, but within the normal range of all
terrestrial herbivores.

GP is equal to the sum of respiration (R = calories used to survive,
including but not limited to BMR) and net production (NP = calories used to
build additional biomass through growth or procreation). The NP of plants
in an ecosystem, the basis of the entire food chain, is called net primary
productivity (NPP).

Net production efficiency (NPE = NP / GP) measures how efficient an animal
is in turning absorbed calories into additional biomass. Observed values in
large mammalian herbivores range from 1.5% for elephants to 12% for moose
(Colinvaux 1986). Hadrosaur diets probably resembled those of elephants, so
let us assume that hadrosaurs had a NPE of 2%, also a limiting value. If
hadrosaur GP was 1.6 x 10^7 kcal/yr, and NPE was 2%, then NP was 3.2 x 10^5
kcal/yr.

Vent rate (V) is the amount of fecal matter, including methane gas,
produced by an animal. V = GP - IB (the total amount of food absorbed into
the bloodstream subtracted from total calories ingested). Our hypothetical
8,000 kg warm-blooded hadrosaur then vents (4.0 - 1.6) or 2.4 x 10^7
kcal/yr. As 6,7 x 10^5 kcal of carbohydrate is one metric tonne of dry mass
(Colinvaux 1986), each hadrosaur leaves behind 36 metric tonnes of dry
fecal matter each year. A significant portion of this is would be gas.


Where the Hadrosaurs Play

We've guessed that a 8,000 endothermic hadrosaur eats 4.0 x 10^7 kcal/yr.
How far must it roam to find this food?

Herbivores rarely eat more than an ecosystem's NPP, for to do so would
irreparably harm the food sources. They eat at most only the extra biomass
produced above maintenance levels.

Beland and Russell (1978) estimate the NPP of the late-K forest that stood
where we now find Dinosaur Provincial Park, Alberta, to be 1.48 x 10^3
kcal/m^2/yr. The NPP of a modern deciduous ecosystem, White Mountain
National Forest, is 4.68 x 10^3 kcal/m^2/yr (Gosz 1978). We will adopt the
first figure for our calculations as it is the most conservative.

Modern African ungulates have exploitation efficiencies (EE = the
percentage of NPP eaten in a given area) of 28-60%. Domestic range land
mammals vary from 30-45% and elephants use only 10% of available NPP. An
estimate of 20% for hadrosaurs would probably be a fair guess.

A hadrosaur requiring 4.0 x 10^7 kcal/yr in Beland and Russell's forest and
harvesting 20% of the available NPP needs to cover 135,000 m^2 each year.
However, we must factor in other species that compete for that food.
Brett-Surman (1979) estimates hadrosaurs made up 75% of herbivore biomass,
Farlow (1976) 69%, and Beland and Russell (1978) 53%. If we use the middle
value, we have to expand each hadrosaur's range to 200,000 m^2 to
compensate, or 4.0 x 10^6 m^2 for a herd of 20 adult animals.

Of course this is an average range based on Beland and Russell's relatively
NPP-impoverished environment. In summertime Alaska's productive forests,
ranges would be smaller; likewise, winter ranges would be larger for active
animals. Ectotherms need about 25% of the resources and space that
warm-blooded creatures do. Our result is also based only on the lowest
estimated energy requirements. Actual ranges would be larger in healthy,
growing populations.




Laurie Nyveen                                  lawrence@dsuper.net
__________________________________________________________________
Editor, Netsurfer Digest - <http://www.netsurf.com/nsd/index.html>
DNRC Minister of Adding "ue" to Words That End in "log"
"All we are, basically, are monkeys with car keys."
                             - Grandma Woody (Northern Exposure)